首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2738篇
  免费   122篇
  国内免费   8篇
化学   2199篇
晶体学   26篇
力学   15篇
数学   156篇
物理学   472篇
  2023年   12篇
  2022年   22篇
  2021年   33篇
  2020年   44篇
  2019年   52篇
  2018年   38篇
  2017年   35篇
  2016年   81篇
  2015年   73篇
  2014年   96篇
  2013年   132篇
  2012年   177篇
  2011年   195篇
  2010年   93篇
  2009年   99篇
  2008年   203篇
  2007年   200篇
  2006年   185篇
  2005年   154篇
  2004年   151篇
  2003年   109篇
  2002年   96篇
  2001年   46篇
  2000年   28篇
  1999年   25篇
  1998年   30篇
  1997年   22篇
  1996年   28篇
  1995年   25篇
  1994年   25篇
  1993年   13篇
  1992年   24篇
  1991年   15篇
  1990年   27篇
  1989年   26篇
  1988年   15篇
  1987年   16篇
  1985年   19篇
  1984年   23篇
  1983年   15篇
  1982年   23篇
  1981年   17篇
  1980年   11篇
  1979年   16篇
  1978年   16篇
  1977年   11篇
  1976年   12篇
  1975年   10篇
  1974年   8篇
  1973年   11篇
排序方式: 共有2868条查询结果,搜索用时 15 毫秒
991.
A 3D-Mössbauer Spectroscopic Microscope is developed to evaluate Fe impurities in multi-crystalline Si solar cells, which combines the Mössbauer spectroscopic microscope with a scanning electron microscope (SEM), an electron beam induced current (EBIC), an electron backscatter diffraction (EBSD), and an electron energy analyzer (HV-CSA). In addition, a new moving-coil-actuator with a liner encoder of 100 nm-resolution is incorporated for the operations with both a constant velocity and a constant acceleration mode successfully with the same precision as that obtained by the conventional transducers. Furthermore, a new multi-capillary X-ray lens is designed to achieve a γ-ray spot size less than 100 μm in diameter. The new microscope provides us to investigate the space correlation between Fe impurities and the lattice defects such as grain boundaries in multi-crystalline Si solar cells.  相似文献   
992.
993.
The reactions of benzonitrile in supercritical methanol, ethanol, and 2-propanol were investigated under non-catalytic conditions. In supercritical methanol, benzonitrile was converted to methyl benzoate in high yield. The esterification reaction also occurred in supercritical ethanol to afford ethyl benzoate in moderate yield. The esterification could occur via a route analogous to the Pinner reaction. On the other hand, benzonitrile in supercritical 2-propanol yielded no ester. Benzyl alcohol was the major product in supercritical 2-propanol. We investigated the reaction of the CN bond in supercritical 2-propanol. In supercritical 2-propanol, N-benzylideneaniline was transferred to the reduction product (N-benzylaniline) and hydrolysis products (benzyl alcohol and aniline). The hydrolysis reaction was restricted when the reaction was carried out in supercritical 2-propanol with a low water content. This indicates that the water in the 2-propanol acts as a reagent for the hydrolysis of the CN bond. These results suggested the following reaction process: C6H5CN→C6H5CHNH→C6H5CHO→C6H5CH2OH.  相似文献   
994.
Novel continuous flow carboxylation system was constructed using pressurized CO2 at 6.5 MPa and immobilized enzyme, Bacillus megaterium PYR 2910 decarboxylase. The decarboxylase catalyzed the backward reaction, carboxylation, to convert pyrrole to pyrrole-2-carboxylate at the improved space-time yield by 25 times compared to the corresponding batch reaction.  相似文献   
995.
996.
997.
Synthesis and isolation of the stable diaryldibromodisilene, Bbt(Br)SiSi(Br)Bbt, has been accomplished for the first time. The dibromodisilene underwent substitution reactions with organometallic reagents on the low-coordinated silicon atom to afford the corresponding substituted disilenes. Furthermore, the reaction of 1 with t-BuLi afforded the corresponding 1,2-diaryldisilyne, BbtSi[triple bond]SiBbt, the characters of which were revealed by spectroscopic and crystallographic analyses.  相似文献   
998.
Although most fulgides show their photochromism in the solid state, crystal structure changes accompanying the photochromism have not been previously observed. The photochromic reactions have been so far considered to take place on surfaces or at defects of the crystals or to proceed with destruction of the crystals. In this study we have succeeded in observing crystal structure changes accompanying the photochromism of fulgides using X-ray diffraction analysis. Detection of the photoproducts in the crystal structures was not possible when the single crystals of the fulgides were irradiated with steady UV light. Two-photon excitation by pulsed laser light was essential to produce a sufficient amount of the photoproducts without significant deterioration in the quality of the crystals.  相似文献   
999.
Iron peroxide species have been identified as important intermediates in a number of nonheme iron as well as heme-containing enzymes, yet there are only a few examples of such species either synthetic or biological that have been well characterized. We describe the synthesis and structural characterization of a new series of five-coordinate (N4S(thiolate))Fe(II) complexes that react with tert-butyl hydroperoxide ((t)BuOOH) or cumenyl hydroperoxide (CmOOH) to give metastable alkylperoxo-iron(III) species (N4S(thiolate)Fe(III)-OOR) at low temperature. These complexes were designed specifically to mimic the nonheme iron active site of superoxide reductase, which contains a five-coordinate iron(II) center bound by one Cys and four His residues in the active form of the protein. The structures of the Fe(II) complexes are analyzed by X-ray crystallography, and their electrochemical properties are assessed by cyclic voltammetry. For the Fe(III)-OOR species, low-temperature UV-vis spectra reveal intense peaks between 500-550 nm that are typical of peroxide to iron(III) ligand-to-metal charge-transfer (LMCT) transitions, and EPR spectroscopy shows that these alkylperoxo species are all low-spin iron(III) complexes. Identification of the vibrational modes of the Fe(III)-OOR unit comes from resonance Raman (RR) spectroscopy, which shows nu(Fe-O) modes between 600-635 cm(-1) and nu(O-O) bands near 800 cm(-1). These Fe-O stretching frequencies are significantly lower than those found in other low-spin Fe(III)-OOR complexes. Trends in the data conclusively show that this weakening of the Fe-O bond arises from a trans influence of the thiolate donor, and density functional theory (DFT) calculations support these findings. These results suggest a role for the cysteine ligand in SOR, and are discussed in light of the recent assessments of the function of the cysteine ligand in this enzyme.  相似文献   
1000.
In order to develop a highly ordered polymer dopant to improve the physical properties of polymer materials for microsystems, a novel supramolecular inclusion complex (IC) of chiral bornyl 4-(6-acryloyloxyhexyloxy) phenyl-4'-benzoate ( BAPB) threaded with beta-cyclodextrins (beta-CDs) was synthesized. The inclusion complex was identified using Fourier transform infrared (FTIR), UV, 13C cross-polarization/magic-angle spinning (CP/MAS) NMR, 1H NMR, and X-ray diffraction (XRD). The construction of the fibrous self-assembled inclusion complex was confirmed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The highly ordered polymerized inclusion complex beta-CD-BAPB revealed significant birefringence and was confirmed using polarized optical microscopy (POM). Polymerization of self-assembled nanofibrous monomers with methyl methacrylate was carried out, and the distribution of the nanofibers in the polymer matrix was confirmed using POM. This investigation demonstrates a novel method for the fabrication of polymeric nanofibers with highly ordered, self-assembled functional monomers. The polymeric nanofibers are expected to improve the physical properties of polymer films in the field of microelectric and micromachine systems (MEMS).  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号